How to render a 30K image with cycles?
With very large image resolutions Blender can run out of RAM. The operating system will then start using the hard disk for memory which is slow and will cause low CPU usage (swapping).

To reduce memory usage for very high resolution renders:


Enable Save Buffers in the Performance panel so Blender does not keep the full render in memory during render
Render from the command line so no display buffer must be kept in memory
Output to an OpenEXR file so one tile can be written at a time